Say I wanted a dark skinned doll with tan lines...

Would i get a light skin doll and send it off to be painted, or would it be easier to get a dark skinned doll and have the lighter parts painted on?

I would think painting the lighter tan lines on a dark skinned doll would be the easier of the two. That's how they did it on the old Malibu Barbies.

The tan lines generally won't appear on jointed sections (where paint is most likely to flake or rub off), so a darker doll with the lighter lines painted on is probably the best long-term option.